Conventionally, a method for obtaining images is known by which image taking processing is performed by an X-ray Computed Tomography (CT) apparatus while using multi different levels of X-ray tube voltages. When two mutually-different levels of X-ray tube voltages are used, the method may be called a “dual-energy CT” method. In “dual-energy CT”, two projection data obtained from two mutually-different levels of X-ray tube voltages are separated into projection data (line-integrated data) each corresponding to respective predetermined two basis materials, and from each of the two separated data, image based on the abundance ratio of the basis materials (basis material image) is reconstructed, the above-described which applied technology is known. According to such applied technology, it is possible to obtain various types of images such as a monochromatic X-ray image, a density image, an effective atomic number image, and the like, by performing a weighted calculation while using the two basis material images.
The applied technology described above is effective in correcting artifacts caused by beam hardening. However, besides the artifacts caused by beam hardening, there are other various artifacts such as those caused by a degradation in the precision level of the projection data due to highly-absorbent materials and those caused by scattered rays.
In particular, artifacts often occur due to a degradation in the precision level of the projection data caused by highly-absorbent materials. The reason can be explained as follows: When a material (e.g., metal) having a large linear absorption coefficient is present in an image taking target, the count of a detector shows a very small value during an image taking processing using a low level of X-ray tube voltage, and it is therefore not possible to obtain proper projection data. In that situation, it is not possible to properly obtain the projection data of the basis materials. As a result, the acquired monochromatic X-ray image will have an artifact where, for example, information in the surroundings of the highly-absorbent material is missing. According to the applied technology described above, it is not possible to generate a monochromatic X-ray image from which the impacts of the artifacts other than the artifacts caused by beam hardening are also eliminated.